Magistra dicit coniunctionem saepe inter duas sententias poni.

Because after dicit, Latin often uses an indirect statement construction: accusative + infinitive.

So in:

Magistra dicit coniunctionem saepe inter duas sententias poni.

the word coniunctionem is not the direct object of dicit in the ordinary English sense. Instead, it is the subject of the infinitive poni inside the indirect statement.

A literal breakdown is:

• Magistra dicit = The teacher says
• coniunctionem poni = that a conjunction is placed

In English, we use that + finite verb. In Latin, this is often replaced by accusative + infinitive.

For the same reason: this is an indirect statement after dicit.

Latin regularly uses:

• a verb of saying, thinking, knowing, perceiving, etc.
• followed by an accusative subject
• and an infinitive verb

So instead of saying something like Magistra dicit quod coniunctio... ponitur, classical Latin normally says:

Magistra dicit coniunctionem... poni.

Here poni is the present passive infinitive of pono, ponere.